1. Field of the Invention
The invention relates to a method for recovering rhodium from residues of the oxo synthesis and/or hydroformylation.
2. Discussion of the Prior Art
Besides cobalt, rhodium catalysts are increasingly used in oxo synthesis. After the distillative separation of the desired oxo synthesis products, rhodium is present in soluble form in the remaining high-boiling distillation residues.
Numerous processes are known for the recovery of this valuable metal. Thus, for example, German Offenlegungsschriften Nos. 2 448 005 and 2 614 799 describe methods for the separation of rhodium-phosphine-carbonyls from rhodium salt solutions resulting at the treatment of distillation residues of hydroformylation mixtures with acids and peroxides.
According to German Offenlegungsschrift No. 2 448 005, the salt solutions are reacted in the presence of a water soluble organic solvent with halogen hydracids or alkali metal halides, phosphines and carbon monoxide and the precipitating rhodium phosphine carbonyls separated, and according to German Offenlegungsschrift No. 26 14 799 they are treated with a cation exchanger for the separation of rhodium ions.
According to the method of German Auslegeschrift No. 2 637 262, a hydrogen stream can be introduced into the byproducts containing the rhodium catalyst for removing organic substances, the residue containing the rhodium in high concentration can be dissolved in an inorganic acid, and the resulting water soluble rhodium compound can be processed or re-used as catalyst.
German Auslegeschrift No. 1 295 537 describes a process for recovering rhodium as metal by blowing water vapor into the hydroformylation mixture.
In the method known from German Offenlegungsschrift No. 2 502 233, the raw products of the hydroformylation or the secondary fractions of the distillation containing rhodium and triphenyl phosphine are treated with a strong acid and an aldehyde and thereby a solid rhodium complex and an aqueous solution containing the triphenyl phosphine obtained.